Engine systems may include boosting devices for boosting the torque and peak power output by an internal combustion engine. By compressing the intake air, the boosting device increases the mass airflow into the engine, which in turn permits a greater amount of fuel to be combusted on each ignition event. One example of a boosting device is a turbocharger wherein a compressor positioned in an intake passage of the engine is mechanically coupled to an exhaust turbine via a shaft. The turbine is spun using exhaust energy which in turn drives the compressor.
Turbochargers may also be configured with electric assistance wherein a motor/generator is coupled to the shaft (herein also referred to as an eTurbo). The eTurbo typically runs on an existing 48V mHEV architecture of the engine with a 48V belt integrated starter generator (BISG), a 48V battery, and a 48V/12V DC-DC converter. The electric assistance can provide improved transient engine response by motoring the turbocharger shaft during conditions when the turbine speed is low (due to low exhaust flow), thereby reducing turbo lag.
Turbochargers, however, have hardware limits. Consequently, the output of turbochargers (both traditional and electric-assisted configurations), may be limited by speed and temperature constraints. For example, there may be a maximum turbocharger shaft speed, which could be violated under high engine load or when operating a vehicle at high altitude. If the shaft speed limit is exceeded, a magnet coupled to the shaft may be degraded, causing the output of the turbocharger to drop. Therefore, turbocharger speed limits cannot be exceeded due to the potential for substantially immediate mechanical degradation. Current control systems may address this issue by clipping the maximum boost pressure when such a constraint violation is anticipated. Additionally, airflow actuators may be adjusted to reduce the boost pressure, such as by opening a wastegate and/or a compressor recirculation valve. However, the drop in boost output below the driver demanded boost pressure may result in a noticeable under-delivery of torque demand, and a drop in vehicle driveability. In addition, the vehicle operator's drive experience is degraded. Still other approaches may be used to maintain the turbocharger speed or temperature within limits, with some margin, so as to avoid reducing the useful life of the turbocharger.
One example approach is shown by Kees et al. in U.S. Pat. No. 9,677,486. Therein, when a turbocharger speed reaches a limit, valves of the deactivated cylinders are opened to permit flow of air through the cylinders. In still other examples, regenerative braking via the motor coupled to the shaft of an eTurbo is used to reduce the turbine speed. The braking energy is recuperated by operating the eTurbo motor as a generator and the recuperated energy is stored in an energy storage device coupled to the motor, such as in a battery.
However, the inventors herein have recognized potential issues with such systems. As one example, the amount of regenerative braking that can be used to reduce turbine speeds may itself be limited. In particular, if the storage device (e.g., a Li ion battery) coupled to the eTurbo motor is already at a higher than threshold state of charge (e.g., fully charged), it may not be able to accept further electrical energy. As such, overfilling can damage the battery. If the waste-gate is opened to rapidly reduce the turbine speed, the boost energy is dumped or wasted and cannot be harvested.
In one example, the issues described above may be addressed by a method for a boosted engine, comprising: responsive to imminent over-speeding of a turbocharger shaft while a system battery is at a higher than threshold state of charge, applying negative torque from an electric motor onto the turbocharger shaft while concurrently applying positive torque from a belt-integrated starter generator (BISG) onto an engine crankshaft; and reducing engine fueling to maintain overall engine torque output. In this way, turbocharger shaft speed can be controlled without overfilling a system battery and while harnessing a larger portion of the braking energy.
As one example, an engine system may be configured with an electric turbocharger having an electric motor coupled to a turbocharger shaft. During an operator pedal tip-in event, to expedite boost delivery, an exhaust waste-gate may be closed and/or a positive torque may be applied on the shaft via the electric motor. If the shaft speed increase is such that an over-speed condition is imminent, negative torque may be applied by the electric motor to slow the shaft. The negative torque may be recuperated by charging a system battery, specifically, a 48V system battery coupled to the vehicle's driveline. Charging may be continued until the battery is at a threshold state of charge (SOC), such as at 95% SOC. Above this level, further charging of the battery can cause overfilling of the battery, which can degrade the battery's performance. Once the battery has reached the threshold SOC, a contactor coupling the battery to a 48V line of the vehicle's electrical system can be opened, thereby disabling further charging of the battery by the e-turbo motor. Thereafter, negative torque applied by the electric motor for shaft speed control may be recuperated by driving a BISG coupled to the driveline. Specifically, the electrical power generated by the electric motor while decelerating the shaft is used to apply a corresponding amount of positive BISG torque on the engine (specifically, on the crankshaft). The torque applied by the BISG may be based on the additional braking required after absorbing torque to charge the battery. The torque absorbed at the BISG may then be used to propel the vehicle. At the same time, engine fueling may be reduced as a function of the BISG torque so as to reduce the engine torque contribution to the driveline and maintain a net wheel torque.
In this way, the braking (negative) torque applied by the electric boost motor to control the turbocharger shaft speed may be used to generate electrical energy that is shared with a 48V distribution box of the vehicle's 48V electrical architecture. When a system battery is full, electrical energy is drawn from the 48V distribution box, as it is generated via the electric motor, to operate the BISG. Additionally, one or more 12V electrical loads may also draw a portion of the electrical power generated by the electric motor during the shaft speed control. By using electrical power generated during turbocharger shaft braking to operate a BISG, shaft speed can be controlled without overfilling a charge sensitive system battery, such as a 48V battery of a hybrid vehicle's driveline. The technical effect of absorbing torque at a BISG is that braking energy may be recuperated through the vehicle's driveline, instead of through the battery. This allows any excess torque generated after the system battery is charged till a threshold SOC to be used to propel the vehicle, instead of being wasted. In addition, turbocharger shaft speed may be controlled with reduced need for waste-gate opening. For example, an exhaust waste-gate may be opened later (e.g., at a higher turbine speed) and/or by a smaller amount. The reduced need for waste-gate opening improves the overall boost response. Overall, violation of a turbocharger speed limit can be avoided, while also reducing overcharging of a charge sensitive storage device.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.